Two elderly men with grey hair hold a stash of small bills as they guard a narrow staircase next to a pharmacy tucked inside a three-storey building, well hidden down a dark alley.

They charge every man who enters two yuan (HK$2.50). The stench of cheap tobacco and urine clings to the stairs that lead to the musty corridor of an old hostel.

A skinny man in his 60s slowly zips up his trousers as he walks out of one of the rooms.

It isn't yet noon.

A row of yellow doors, some open, some closed, line the right-hand side of the corridor. There is no need for a reception desk. Only two types of people come here and they know what they want; men seeking cheap sex and women desperate for money.

The scene is typical of the mainland's 10-yuan shops. These cheap brothels are common in less developed cities and townships, catering for the grass-roots of society, people who can't afford to pay for better. The women are ageing sex workers gone to seed from years of toiling in low-end brothels.

Like everything else in the mainland, inflation has taken hold and while the 10-yuan name remains, services are more expensive. Sex costs between 25 and 40 yuan - less than the price of a cinema ticket - with discounts for loyal customers.

Shan, a troubled young woman who is approaching 30 years of age, is one of the very few younger workers at one 10-yuan shop in her native Guangxi Zhuang autonomous region.

Not pretty enough to earn money in a higher class establishment, she has been working in sex shops for three years. While she occasionally gambles, most of the 50 to 100 yuan she earns from each client goes towards sending 5,000 yuan per month to help pay for her 15-year-old sister's leukaemia treatment.

"Her lifeline is being fuelled by money. Our family has already poured over a million yuan into her treatment. Otherwise why would I do this for a job?" Shan said. "We used to have a car, a house and everything, life was good.

"Before I started three years ago, I lingered outside over 30 times, not daring to walk in. But I needed to figure out a way to get more money, so I forced myself in.

"I cried every time after finishing a shift in my first two weeks. It's not something I could ever get used to, but one must bow to the reality," Shan says.

With her health frail, Shan hopes to save enough money to leave the trade and set up an online shop in a few years.

The majority of sex workers in 10-yuan shops are aged 40 or over, with some in their 50 and 60s. Uneducated and with little bargaining power, it is estimated that at least 90 per cent carry sexually transmitted diseases, as the use of condoms is negotiable, advocates for sex workers' rights say.

"They are less conscious of the risks of HIV/Aids, which means they can easily be pressed into not using protection. Some even thought they could get a government subsidy after contracting Aids," said a rights advocate who asked not to be identified.

According to The World Health Organisation and a report by New York-based Human Rights Watch released in May, there are four to six million sex workers on the mainland, although some estimates put the figure as high as 10 to 20 million and say they account for 6 per cent of its gross domestic product.

Men outnumber women on the mainland thanks to the distortions of the one-child policy and a preference for male babies. The distorted sex ratio has created enormous demand for cheap sex among hundreds of millions of male migrant workers.

A survey in 2009 found about 30 per cent of married male rural migrants had visited a prostitute.

This Sunday Morning Post reporter had to shake off a police tail before making her way to the 10-yuan shop in a small, rural community under Bobai county, Guangxi. The name of the township, nearly 300 kilometres from the regional capital, Nanning , is not being disclosed in order to protect the sex workers there.

For a non-local, the hostel is almost impossible to find. One must twist and turn through a wet market where most vendors sell lychees, the local speciality, for 10 yuan per half kilo.

At the 10-yuan shop, most of the customers who have paid the two yuan for admittance are elderly men, construction labourers and migrant workers, with a few curious young rural men mixed in.

A closed door means business is in session, but the customer is welcome to peep into the open ones to consult with the women inside. Undecided customers linger in the corridor, eyeing the ladies in heavy make-up, wearing hot shorts and low-cut tops as they watch television or do cross-stitch on their beds.

The rooms are mostly six square metres in size, furnished with a built-in squat toilet, double bed, pillows, cupboards and a television, with a hi-fi system in some. Rent is 40 to 50 yuan per day. Each of the storeys has about 15 such rooms.

"No more room is available on the second floor. Check out the third to see if there are vacant ones. I heard a girl just moved in today on the third floor," said one sex worker in her 40s who said she came from the impoverished Guizhou province. As she sits on a stool by her open door, her eyes never move from the embroidery as she stitches Chinese characters meaning "may peace be with the entire family" and shares her experiences with a woman she assumes is a fellow sex worker looking for somewhere to operate. On her neatly laid-out bed are three sachets of condoms and a tub of lubricant.

"You go ask the lady-boss here for a room," she says, though she refuses to give out the hostel owner's number.

"Business is not bad, at least six or seven [customers] a day…You should use a condom, but it's up to you," she says.

She hints that the secluded hostel is safe for now, as long as nobody entertains clients outside the premises, or talks to outsiders - especially journalists.

Policemen and journalists are the two groups of people they fear most, even more so since the middle of last month, when Ye Haiyan , an advocate for sex workers' rights, was released from detention, attracting considerable media attention.

"The police have been tailing journalists until they leave Bobai. If sex workers were found to be talking to a reporter, they would be arrested and fined and the whole sex shop would be raided and shut down. Many would lose their jobs," said a local rights activist who claimed to have been threatened by a senior police inspector.

The instructions for such action would have come from the state security personnel in Yulin city and been passed down to officials across its jurisdiction, including Bobai county and its subsidiary townships, according to the activist.

"Sex shop owners are also [meant] to make sure all working girls remain indoors," the activist added.

The sex trade is rampant in Bobai. Every evening, dozens of fliers advertising for call girls are shoved under the doors of hotel rooms. The minimum charge is 150 yuan for 20 minutes with the girls, who are in their 20s and 30s and come escorted by bouncers. They can earn 6,000 to 10,000 yuan per month. Most of the older, grass-roots sex workers can earn just half that or less.

Ye says there are three kinds of sex worker on the mainland. They are classified by "legality" - despite the fact that all kinds of prostitution is officially illegal.

Those referred to as "illegal" are the grass-roots workers like Shan, who are often exploited and extorted in police crackdowns because they don't have the income to pay a "bribe for safety". Those who have connections with the police and suffer only occasional raids are considered "semi-legal".

"The 'legal' ones are usually beautiful and young women with very high income catering for high-end customers. They drive luxurious cars and carry fancy handbags. It usually takes a while to figure out they are in fact sex workers," Ye said.

Rampant corruption leaves local sex workers in Bobai county feeling like poultry, locals say, with the police constantly holding knives to their throats.

Arrested sex workers face a fine of up to 5,000 yuan and 15 days in detention for their first offence. For repeat offenders, the punishment is up to two years in detention.

A former owner of a low-end brothel who used to hire six women said she had left the business recently as it was impossible to survive in Bobai without offering "protection money".

She spoke of one of her former girls, Hongjuan, a mother of three from Hubei province who was forced to flee her home after being abused by her husband.

"She's very hard-working. She could rise at noon and work until 4 am, taking 20 customers a day at times but earning up to 10,000 yuan a month," the brothel owner said.

Once, Hongjuan was beaten by one of her clients in a hotel so she asked the brothel owner for help. Her boss ended up calling the police.

"She wouldn't get into a police vehicle, but also wouldn't tell me why," the brothel owner said.

"That evening, she collapsed and cried. She said the police, during the day, were the ones who extorted her. She said she would rather die than get into one of their cars," the owner said.

"In Bobai, it's a world where man eats man. Everything is about money. There is no mercy. Police are dogs. They charge fines of whatever amount they like without giving receipts. It's even easier penalising prostitutes than traffic offenders."